1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and particularly to a nitride semiconductor light emitting device having a tunnel junction.
2. Description of the Background Art
Regarding a nitride semiconductor light emitting diode device having its side of a p-type nitride semiconductor layer that is a light extraction side, it has conventionally been required that a p-side electrode formed on the p-type nitride semiconductor layer satisfies the following three conditions.
A first condition is that the p-side electrode has a high transmittance for the light emitted from the nitride semiconductor light-emitting diode device. A second condition is that the p-side electrode has a resistance and a thickness that allow injected electric current to be diffused sufficiently within a plane of a light-emitting layer. A third condition is that the p-side electrode has a low contact resistance with respect to the p-type nitride semiconductor layer.
In the case where the side of the p-type nitride semiconductor layer of the nitride semiconductor light emitting diode device is the light extraction side, the p-side electrode formed on the p-type nitride semiconductor layer is conventionally a semi-transparent metal electrode that is formed as a film of a metal such as palladium or nickel having a thickness of approximately a few nm to 10 nm and formed on the entire surface of the p-type nitride semiconductor layer. Such a semi-transparent metal electrode, however, has a low transmittance of approximately 50% for the light emitted from the nitride semiconductor light emitting diode device and thus decreases the light extraction efficiency, resulting in the problem that a high-luminance nitride semiconductor light emitting diode device is difficult to obtain.
Accordingly, a high-luminance nitride semiconductor light emitting diode device is manufactured having, instead of the semi-transparent metal electrode formed as a film of a metal such as palladium or nickel, a transparent and electrically conductive film made of ITO (Indium Tin Oxide) and formed on the entire surface of the p-type nitride semiconductor layer so as to improve the light extraction efficiency. Regarding the nitride semiconductor light emitting diode device having such a transparent and electrically conductive film formed therein, the issue of the contact resistance between the transparent and electrically conductive film and the p-type nitride semiconductor layer is improved by heat treatment or the like.
Patent Document 1 (Japanese Patent Laying-Open No. 2002-319703) discloses a nitride semiconductor light emitting diode device having a group III nitride semiconductor multilayer structure formed on a substrate, and the multilayer structure has at least a first n-type group III nitride semiconductor multilayer structure, a p-type group III nitride semiconductor multilayer structure and a second n-type group III nitride semiconductor layered structure. An n-type group III nitride semiconductor layer in the first n-type group III nitride semiconductor multilayer structure is provided with a negative electrode, an n-type group III nitride semiconductor layer in the second n-type group III nitride semiconductor multilayer structure is provided with a positive electrode, and a tunnel junction is formed by the n-type group III nitride semiconductor layer in the second n-type group III nitride semiconductor multilayer structure and a p-type group III nitride semiconductor layer in the p-type group III nitride semiconductor multilayer structure.
In the nitride semiconductor light emitting diode device disclosed in Patent document 1, the positive electrode is formed at the n-type group III nitride semiconductor layer in the second n-type group III nitride semiconductor multilayer structure, and the n-type group III nitride semiconductor has a carrier density that can be easily increased as compared with the p-type group III nitride semiconductor. Therefore, the contact resistance can be reduced as compared with the conventional structure having its positive electrode formed at the p-type group III nitride semiconductor layer, the driving voltage is lower and larger output drive can be achieved. Further, since the heat generation of the positive electrode that is one of factors of a failure of the nitride semiconductor light emitting diode device is reduced, the diode device is regarded to be able to have improved reliability.